A guy at a local pro shop told me to put them in every other location within the sweetspot to avoid too-lumpy of a stringbed. I would imagine that this configuration wouldn't extend string-life at all, since the un-protected intersections would still rub just as much and break.

Do you need to put them at every intersection in the sweet spot to gain their benefits?

I also have found that these savers do not actually stop the strings from moving so much as they stop them from biting into eachother.